1. Technical Field of the Invention
The present invention relates to a three-dimensional fiber reinforced implant, particularly a vertebral cage for insertion between two adjacent vertebra.
2. Prior Art
Fiber reinforced implants are well known. U.S. Pat. No. 5,429,863 for example discloses a vertebral implant cage, that is fabricated from a block, which is a fiber reinforced composite structure. Carbon fibers are located in every part of the block and randomly interlocked. The cage may have the shape of a cylindrical rod and is provided with cavities which are filled with bone material and is designed to be inserted between adjacent vertebrae.
U.S. Pat. No. 5,906,616 discloses a conically-shaped fusion cage provided with a thread formed as a part of an external conical surface. Apertures provide for bone growth between the engaged vertical bone and bone material packed within the cage.
U.S. Pat. No. 5,968,098 discloses a fusion cage having a generally elliptical cross-section. It includes an entry end portion, a trailing end portion and a thread as a part of an external conical surface. The cage is preloaded with bone material and inserted into the desired surgical location with well known surgical instruments.
A fusion cage formed of radiolucent material is also disclosed in EP-A-0 307 241. The cage has a roughened outer surface for receiving bone in-growth and end faces with means securing it on a tool for insertion on the desired site of the vertebrae.
The role of a vertebral implant is to stabilize a vertebral segment and to bear load while the surrounding bone consolidates, taking over the mechanical function with a viable bone fusion. On one hand the implant must be robust enough to bear rotation at insertion, and axial load, sheer and fatigue during weight bearing. On the other, the implant must provide enough space for bone graft to grow through or around the device. Thus cage designers are faced with a trade off what makes the implant bear load, and the bone ports which must carry enough bone tissue required for bone consolidation. Furthermore, it has been postulated that stress shielding in an implant may prevent fusion of viable bone through the implant, and strength and stiffness should be as close to the surrounding bone tissue as possible.
Several materials are used for inter-body cages and most commonly are Titanium Alloy, PEEK as well as carbon composite. Titanium, while certainly strong enough for the application, has the disadvantage of being a radiographically opaque, making it impossible to visualize if bone has grown through the cage with standard x-ray. It is also known that titanium also produces artificiats for other radiographic examinations such as C.T. or MRI.